1. Field of the invention
The present invention relates to a sewing machine comprising a cloth transport mechanism associated with a device for adjusting the amplitude and the direction of the feed of the cloth, this mechanism comprising a drive shaft and a cloth transport member whose two opposite ends are connected kinematically to this drive shaft by two connecting rod and lever mechanisms, one of which defines a first displacement trajectory in a direction substantially parallel to a support plane for the cloth, while the other defines a second displacement trajectory in a direction substantially perpendicular to this plane, the said adjusting device comprising a slideway having two tracks in the form of arcs of concentric circles centered on a trajectory described by a pivotal connection between two connecting rods, this slideway being pivotably mounted about an axis passing through the center of this trajectory.
2. Description of the prior art
There already exists on the market sewing machines whose transport mechanism is equipped with such an adjusting device for its amplitude and for its direction of cloth feed. In these machines sold under the Trade Mark ELNA, types 5000, 6000 and 7000, the angular position of the control cam is determined by means of a stepper motor.
A problem posed by this adjusting device arises from the guidance of the axis of pivoting between the connecting rod and the short connecting rod along a slideway formed by two tracks in the form of arcs of concentric circles. The engagement between such a cam profile and a follower concentric with the above mentioned axis of pivoting does not permit the carrying out of too tight an adjustment because of the risk of seizure between the follower member and the slideway. To avoid this disadvantage, a small play exists between the slideway and the follower member mounted about the axis of pivoting. This play constitutes a source of imprecision in the path of transport of the cloth, in particular in the case of changing the direction of this transport. This results, in particular in the case of the sewing of a buttonhole, in adjacent edges possibly having different lengths. In effect, as small as this error may be, by multiplying itself by the number of stitches, it finishes by becoming visible to the eye, especially in the case of a symmetrical pattern of the type of that of a buttonhole.